<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=, initial-scale=1.0">
    <title>Document</title>
    <style>
        .fixd {
            position: fixed;
            left: 50%;
            margin-left: 400px;
            width: 30px;
            height: 40px;
            background-color: rgb(21, 85, 141);

        }

        .w {
            width: 800px;
            height: 30000px;
            background-color: bisque;
            margin: 0 auto;
        }
    </style>
</head>

<body>
    <div class="fixd">
    </div>
    <div class="w">版心盒子</div>
</body>

</html>